The recent literature has shown a correlation between the presence of the estrogen receptor proteins (ERP) in malignancy and the remission of the tumor after endocrine ablation. H-3 labeled estrogens concentrate in those human breast tumors that contain ERP. A gamma emitting estrogen derivative would be useful to determine the presence of ERP in metastatic tumors by external imaging. To this end five compounds were obtained: estradiol (E), hexestrol (HEX), estradiol 17 beta succinyl tyrosine methylester (EST), estradiol 6-(O-carboxymethyl oxime tyrosine methyl ester and 17 alpha ethynyl estradiol (EE). The compounds were iodinated and then analyzed by various chromatographic systems. The in vitro ERP binding affinity of the five compounds before and after iodination was determined by a competitive binding study using H-3 E. E and EE displaced H-3 E equally well (50 percent inhibition at 2.9 by H-3 E) followed by HEX (50 percent inhibition at 6.4 times) and EST (50 percent inhibition at 200 x). In contrast, iodinated E could not displace H-3 E. Iodinated HEX could displace 50 percent of H-3 E at 100 fold excess but iodinated EE displaced H-3 with an affinity equal to E with an affinity equal to E and the parent compound EE. In this respect iodinated EE is the superior derivative. Iodinated HEX showed specific binding to ERP by sucrose gradient centrifugation but all iodinated compounds showed nonspecific binding. This nonspecific binding could be decreased for iodinated HEX and EE by thyroxine and iodoehtynyl cyclohexanol. In agreement with these data iodinated HEX and EE showed high uterine uptake in immature rats. We suggest that the target to non target ratio in vivo can be anticipated by the extent of specific and non specific binding in vitro.